Warning: Invalid target.

Describe the bug: バグの概要

After the GH Action https://github.com/marketplace/actions/stale-branches has opened up an issue, the GH Action https://github.com/marketplace/actions/assign-author is failing to assign the author to this newly opened issue. The Action log is only showing "Warning: Invalid target." without any further details, despite the debug logs were switched on. The dumped payload is showing that the correct issue was in scope.
The action works properly for assigning author to pull requests

To Reproduce: 再現手順

Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior: 期待する動作

Screenshots: スクリーンショット

Operating environment: バグが発生した環境

• Version of software
• OS: [e.g. Windows10]
• Browser: [e.g. Chrome, Safari]
• etc.

Additional context: 補足

[welcome]

🙌 Thanks for opening your first issue here! Be sure to follow the issue template!